This invention relates to a braking device for use in a motor vehicle of the type that includes a vacuum motor having a constant pressure chamber and a variable pressure chamber formed therein, a power piston actuated by the pressure difference between the two chambers, an input rod connected to a brake pedal of the vehicle, and a valve mechanism for controlling the pressure in the variable pressure chamber in response to the input force applied to the input rod. More particularly, this invention relates to a braking device of the type mentioned above which further includes a control valve controlled electrically and remotely through a manual operation and adapted to be capable of controlling the vacuum motor without any need for the brake pedal to be depressed.
There has recently a demand for a enabling a braking force to be applied in a vehicle without any need for the brake pedal to be depressed. For this purpose, Robert R. Hager et al. disclosed in U.S. Pat. No. 3,364,818 a braking device in which pressure applied to a push button or the like arranged at a location near the driver's seat is mechanically transmitted to a relay valve and the pressure thereby generated is supplied to a vacuum motor to generate braking force. The vacuum motor has a shell and a power piston mounted in the shell through a diaphragm and serving to partition the interior of the shell into a variable pressure chamber and a constant pressure chamber. The power piston is displaced by virtue of pressure difference between the variable pressure chamber and the constant pressure chamber, thereby to move an output rod mounted on the power piston in the axial direction. However, in such a conventional braking device, the pressure applied to the push button has a certain special relationship with the braking force to be generated, and it is impossible to control the braking force finely. The characteristic curve (a) in FIG. 14 shows the relationship between pressure on the push button and the vacuum motor driving pressure generated in the relay valve. It is to be noted that this relationship has a hysteresis characteristic. The vacuum motor driving pressure is determined by pressure difference (V.sub.a -V.sub.b), where V.sub.a denotes the degree of vacuum in the constant pressure chamber or in a vacuum source, and V.sub.b denotes the degree of vacuum in the variable pressure chamber. The characteristic curve (b) in FIG. 14 shows the relationship between the driving pressure for the vacuum motor or the brake system and a braking force or brake liquid pressure p to be generated, which also has a hysteresis characteristic. The curve (c) in FIG. 14 shows the relationship between the pressure applied to the push button and the brake liquid pressure. It is to be noted that this relationship has a particular characteristic with a large degree of ineffective input and a large amount of hysteresis because of the combination of the hysteresis factors of (a) and (b). Thus, it is also clear that it is impossible to control the braking force finely.
Japanese Patent Publication No. 48-40265 discloses a control device for controlling a vacuum motor in which a relay valve which generates pressure proportional to the input applied thereto is remotely controlled by utilizing hydraulic pressure to connect a variable pressure chamber and a constant pressure chamber of a vacuum motor to a vacuum source and the atmosphere, respectively, thereby controlling the vacuum motor. However, in this prior art, it is necessary for pipe means to be laid to connect relay valve to an actuating member, for example, a manual brake lever mounted by the driver's seat. The necessity to lay the pipe means in a limited space within the chassis of a vehicle causes various problems in terms of the need to arrange many different items of vehicle equipment.